The present disclosure relates generally to the field of nonvolatile memory devices, and more specifically to an element of a magnetic random access memory (MRAM) device that uses spin torque transfer.
MRAM is a nonvolatile memory technology that uses magnetization to represent stored data. MRAMs are beneficial in that they retain stored data in the absence of electricity. Generally, MRAM includes a plurality of magnetic cells in an array. Each cell typically represents one bit of data. Included in the cells are magnetic elements. A magnetic element may include two ferromagnetic “plates” (or layers upon a semiconductor substrate) each of which has a magnetization direction (or orientation of magnetic moments) associated with it. The two ferromagnetic plates are separated by a thin non-magnetic layer.
More specifically, MRAM cells are often based on a magnetic tunnel junction (MTJ) element (also known as tunnel magnetoresistance (TMR) elements). An MTJ element includes at least three basic layers: a “free layer,” a tunneling barrier layer, and a “pinned layer.” The free layer and the pinned layer are ferromagnetic layers, the tunneling barrier layer is a thin insulator layer located between the free layer and the pinned layer. In the free layer, the magnetization direction is free to rotate; the magnetization of the pinned layer is not. An antiferromagnetic layer may be used to fix, or pin, the magnetization of the pinned layer in a particular direction. A bit is written to the element by changing the magnetization direction of one of the ferromagnetic plates of the magnetic element. Depending upon the orientations of the magnetic moments of the free layer and the pinned layer, the resistance of the MTJ element will change. Thus, the bit may be read by determining the resistance of the magnetic element. When the magnetization of the free layer and the pinned layer are parallel and the magnetic moments have the same polarity, the resistance of the MTJ element is low. Typically, this is designated a “0.” When the magnetization of the free layer and the pinned layer are antiparallel (i.e. the magnetic moments have the opposite polarity), the resistance of the MTJ is high. Typically, this is designated a “1.”
Spin torque transfer (STT) (also known as spin transfer switching or spin-transfer effect) is one technique for writing to memory elements. STT was developed as an alternative to using an external magnetic field to switch the direction of a free layer in the magnetic element. STT is based upon the idea that when a spin-polarized current (most of the electrons of the current have spins aligned in the same direction) is applied to a “free” ferromagnetic layer, the electrons may get repolarized on account of the orientation of the magnetic moments of the “free layer.” The repolarizing of the electrons leads to the free layer experiencing a torque associated with the change in the angular momentum of the electrons as they are repolarized. As a result, if the current density is high enough, this torque has enough energy to switch the direction of the magnetization of the free layer. The advantages of using STT for writing to magnetic elements are known in the art and include smaller bit size, lower number of process steps as compared with other writing techniques, scalability for large arrays, and lower writing current requirement. However, there are also disadvantages to using STT for writing to magnetic elements, as the current density required to switch the direction of magnetization in a free layer in the magnetic element is quite large. The critical current density required to switch the layer is denoted as “Jc.” In a conventional embodiment, Jc may be greater than 1E106 A/cm2.
As such, an improved magnetic element architecture allowing the use of STT is desired.